Dibenzo-18-crown-6

The asymmetric unit of the title compound, C20H24O6, contains two molecules that are identical within standard deviations concerning bond lengths and angles as well as their conformations. In the crystal structure, weak C—H⋯O interactions help to consolidate the packing.

We thank Professor Bill Clegg (University of Newcastle/ Daresbury Laboratory) for collecting the diffraction data and performing the initial processing.

S1. Comment
The relationship between the conformations of crown ethers and their coordinating abilities has been the subject of many crystallographic, spectroscopic, thermochemical and theoretical studies (e.g. Grotjahn et al., 2001;Barranikov et al., 2002;Su et al., 2003). The crystal structures of a large number of complexes containing the title crown ether have been determined, but the structure of the title compound, (I), the free crown ether, has remained undetermined until now.
There are two molecules in the asymmetric unit of (I) ( Fig. 1) with very similar conformations, with both possessing local approximate C 2 symmetry. This is also reflected in the trans-gauche-trans-gauche sequence of conformation angles about the four O-C-C-O bonds in the 18-membered ring. The dihedral angles between the mean planes of the aromatic rings are 65.5 (3)° for the C1 molecule and 66.1 (3)° for the C21 molecule. Otherwise, the geometrical paramaters for (I) may be regarded as normal (Allen et al., 1987).
In the crystal of (I), two weak intermolecular C-H···O interactions (Table 1) may help to consolidate the packing.
There are no π-π stacking interactions in (I), the minimum ring centroid separation being greater than 5.6 Å. The packing ( Fig. 3) for (I) results in (001) pseudo layers of the two asymmetric molecules.

S3. Refinement
The situation of two asymmetric molecules in space group Cc is a suspicious one (Baur & Kassner, 1992) and careful checks for additional or missed crystal symmetry were made, but none was found. No starting models could be established in space groups C2/c, C2/m or any lower symmetry centrosymmetric space groups. Structure solutions in lower-symmetry non-centrosymmetric space groups were easily achieved and could all be transformed to the model described above.
Even with the use of synchrotron radiation, the small crystal size resulted in weak diffraction and a poor data to parameter ratio of 7.6:1. The residuals are also high. Anomalous dispersion was negligible and Friedel pairs were merged before refinement. The hydrogen atoms were placed in calculated positions (C-H = 0.95-0.99 Å) and refined as riding with U iso (H) = 1.2U eq (C).  View of the molecular structure of (I) showing 50% displacement ellipsoids. The H atoms are drawn as spheres of arbitrary radius.

Figure 2
The packing in (I), viewed down [100] with H atoms omitted for clarity. Bonds in the C1 and C21 molecules are coloured red and blue, respectively.
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.50 e Å −3 Δρ min = −0.52 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.067 (7) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.